The paranodal junction (PNJ) is a crucial component of myelinated nerve fibers responsible for attaching the myelin sheath to the axon, restricting short-circuiting of nodal action currents under the myelin sheath, segregating nodal Na channels from juxtaparanodal K+ channels and maintaining axonal domains over time. Long-term stability of this junction depends on the presence of a structural element, the transverse bands, without which the PNJ deteriorates gradually, resulting in progressive neurological impairment and shortened lifespan. PNJs in the PNS also display short-term structural plasticity in response to electrical stimulation that may modulate nodal behavior. In this renewal period, we propose to address three questions as a way of further probing the function of the PNJ and the transverse bands and their role in physiology and pathology: 1. How do PNJs in the CNS, where myelin is thinner and is formed by oligodendrocytes, differ with respect to structure and permeability from those in the PNS, where myelin is formed by Schwann cells? 2. How 'plastic' are PNJs in the CNS; i.e., how do their structure and permeability change with neural activity, and how do those changes affect function? 3. How do genetic defects that cause loss of transverse bands affect the structure and permeability of PNJs and their susceptibility to activity-mediated changes? The proposed studies will make use of new methods, specifically EM tomography and high-pressure freezing, for defining the structure and dimensions of the PNJ. Second, we will make use of fluorescent dextran tracers and electron opaque tracers for comparing the permeability of CNS PNJs with that of PNJs in the PNS. Third, we will examine the effects of different degrees of electrical stimulation in situ on the structure and permeability of PNJs in order to define their responses to normal and abnormal levels of activity. Finally we will carry out parallel structural, permeability and stimulation studies on mutant mice with genetically defective transverse bands in order to determine how these defects modify the structure and permeability of the PNJs and their susceptibility to activity-induced changes. These studies are relevant to genetic and acquired human diseases of myelin in which pathological processes affect transverse bands and the PNJ.